In recent years, with the spread of portable electronic devices such as portable telephones, notebook personal computers and the like, there is a large need for development of compact and lightweight secondary batteries having a high energy density. Moreover, there is a large need for high-output secondary batteries as the batteries for power supplies of motor drives, and particularly for power supplies for transport equipment.
As a rechargeable battery that satisfies such a demand is a lithium-ion rechargeable battery that is one kind of a non-aqueous electrolyte rechargeable battery. This lithium-ion rechargeable battery includes an anode, a cathode, an electrolyte and the like; and a material for which extraction and insertion of lithium is possible is used as the active material that is used as the material for the anode and cathode.
Currently, much research is being performed for various kinds of lithium-ion batteries, and of that research, a lithium-ion rechargeable battery in which a layered-type or spinel-type lithium composite metal oxide is used as the cathode material is capable of obtaining a high 4V class voltage, so application as a battery having high energy density is being advanced.
Currently, as the cathode material for this kind of lithium-ion rechargeable battery, lithium composite oxides such as lithium cobalt composite oxide (LiCoO2) for which synthesis is comparatively easy, lithium nickel composite oxide (LiNiO2) in which nickel that is less expensive than cobalt is used, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese, lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2), and the like are proposed. Of these, lithium nickel cobalt manganese composite oxide is gaining attention as a cathode material that has good charge/discharge cycling characteristics, low resistance, and from which high output can be obtained. Moreover, tests for increasing the performance by introducing various additional elements into this lithium nickel cobalt manganese composite oxide are being performed.
For example, JP 2012252964 (A) discloses being able to reduce the cathode resistance while maintaining the initial discharge capacity, and improve the capacity retention after a cycling test by including at least 0.02 mol % to 1 mol % of calcium and 0.05 mol % or less of magnesium in the lithium nickel cobalt manganese composite oxide. Moreover, JP 2012252964 (A) discloses being able to promote crystal growth during calcination by including 0.08 mol % to 1 mol % of sodium. Furthermore, JP 2012252964 (A) discloses being able to prevent a decrease in crystallinity and a decrease in battery characteristics that is due to that decrease in crystallinity by controlling the amount of SO4 included to be 1% by mass or less.
On the other hand, research is not simply being performed in regard to the introduction of additional elements into the lithium nickel cobalt manganese composite oxide, but research that is focusing on the crystal forms is also being performed.
For example, JP 2003077460 (A) proposes a cathode active material, wherein, of the lithium nickel cobalt composite oxide of the cathode active material that includes lithium niobate, when the X-ray diffraction peak intensity on plane (003) is taken to be I(003), the X-ray diffraction peak intensity on plane (104) is taken to be I(104), and the maximum X-ray diffraction peak attributing to the lithium niobate is taken to be INb, the ratios of these peak intensities are: I(003)/I(104) is 1.6 or greater, and 0.01≦INb/I(003)≦0.03. According to JP 2003077460 (A), when using this kind of cathode active material in a non-aqueous electrolyte rechargeable battery, damage or fire does not occur even when there is internal shorting, so it is possible to improve safety.
JP 2007123255 (A) proposes a lithium transition metal composite oxide that is expressed as Li1+xM1−xO2 (where M is at least one kind of transition metal that is selected from among Ni, Mn, Co, Fe, Cu, Zn, Cr, Ti, and Zr, and 0≦x≦0.15), the amount of acid radical (sulfate radical: SO3, chlorine radical: Cl) included being 1500 ppm or less, the amount of alkali metal (Na, K) included being 2000 ppm or less, and the peak intensity ratio I(003)/I(104) of X-ray diffraction peaks on plane (003) and plane (104) attributed to the hexagonal crystals being 1.4 or greater. Of this lithium transition metal composite oxide, not only is it possible to make the discharge capacity large even when the amount of cobalt included is reduced, but it is also possible to achieve excellent discharge rate characteristics.
In addition, JP H10308218 (A) proposes a lithium composite oxide for which it is possible to obtain both improved thermal stability and charge/discharge cycling characteristics when charging the lithium-ion rechargeable battery by regulating the crystallite size that is calculated from plane (003) using the Scherrer formula and the crystallite size that is calculated from plane (110) using the Scherrer formula to be within a specified range.
In these documents, even though improving the safety and discharge capacity by regulating the peak intensity ratio of specified planes of lithium composite oxide is proposed, improvement of the input/output characteristics of a lithium-ion rechargeable battery has not been studied sufficiently. On the other hand, with the worldwide spread of portable electronic devices and electric automobiles, there is a need for further improvement of input/output characteristics of the lithium-ion secondary batteries that are used in these devices.
Here, the input/output characteristics of a lithium-ion rechargeable battery are known to have a strong correlation with the direct-current resistance (DCIR) that is expressed as the resistance of the overall battery. Therefore, in order to improve the input/output characteristics, reducing the DCIR is important. Particularly, in a state in which the charging depth (SOC) at the end of discharge is low, the DCIR becomes large, so improving the DCIR in this low SOC state is important for improving the battery characteristics.
For example, JP 2005197004 (A) discloses a layered lithium nickel manganese composite oxide that is expressed by the compositional formula: LiaMnxNiyCozO2 (where 0<a≦1.2, 0.1≦x≦0.9, 0≦y≦0.44, 0.1≦z≦0.6, and x+y+z=1), and of which the ratio of the peak intensity (I(003)) on plane (003) of the X-ray diffraction pattern on plane (003) and peak intensity (I(104)) on plane (104) is controlled to be no less than 1.0 and no greater than 1.5, and the specific surface area is controlled to be 0.6 m2/g to 1.5 m2/g. JP 2005197004 (A) discloses that it is possible to obtain this kind of lithium nickel manganese composite oxide by performing calcination of the raw material that has a small particle size for 10 hours to 50 hours at 950° C. or greater, and preferably at 1000° C. to 1100° C.
Moreover, JP 2013051772 (A) discloses a lithium-ion rechargeable battery, of which the cathode active material has hollow structure, and by controlling the ratio (FWHM(003)/FWHM(104)) of the half peak width (FWHM(003)) of the diffraction peak on plane (003) with respect to the half peak width (FWHM(104)) of the diffraction peak on plane (104) to be 0.7 or less, the battery is able to display high output characteristics even in a low SOC state of 30% or less, and in low-temperature environments of −30° C. JP2013051772 (A) discloses that this kind of cathode active material can be obtained by mixing transition metal hydroxide that has been crystallized under specified conditions with a lithium compound, and performing calcination of that mixture in an oxidizing atmosphere for 3 hours to 20 hours at a maximum calcination temperature of 700° C. to 1000° C.
Therefore, by using the cathode active materials disclosed in the patent literature above, it is feasible that together with increasing the output of a rechargeable battery, it is possible to reduce the internal resistance at extremely low temperature (−30° C.). However, in JP 2005197004 (A), there is only an evaluation of the quantitative tendencies on plane (003) and plane (104) using the peak intensities, and there is no quantitative evaluation of the crystal planes important for the input/output characteristics, or a sufficient evaluation of the crystallinity. Moreover, in JP 2013051772 (A), the half peak widths are only used to evaluate the relative crystallinity between crystal surfaces, and even though a cathode active material with the above properties is obtained, it is not possible to obtain a sufficient effect for reducing the DCIR in a low SOC state of 20% or less.